This invention relates in general to hydraulic drive and control systems for process line equipment. More specifically, it relates to an electrohydraulic drive and control system particularly useful for a spooler (also known as a traverse winder or level winder) that both winds and pays out an indefinite length of metallic strand.
In the production of many materials, whether metal, paper, plastic films or otherwise, the product is in the form of a moving strand or web. In the case of a strand, it can be a solid wire, tubing, strip, or a variety of other forms. Processing of the material occurs "on the fly" as it moves through the production equipment. Typically when the processing is complete, the material is wound onto a spool, core, reel or mandrel. In some applications, the material is wound and then later unwound for further processing. Regardless of the nature of the material, its form, or the type of processing, it is always important to control the speed and tension of the material during the processing.
Speed control is important because different materials or operations may require different speeds. A drive system must be able to produce, and/or match, a wide range of line speeds, to adjust the line speed, to jog at slow speeds (with and without tension in the strand), to accelerate and decelerate, and in winding or unwinding to vary the strand speed as a function of the coil diameter. Torque control is also very important in establishing a correct degree of tension in the strand. The drive system can be a master or slave in setting or following the line speed and all following slave drives normally need to operate in a tension control mode on a taut strand.
Tension control is important for many reasons. If it is too high, the strand may break or be damaged. If it is too slack, various operations may not be performed effectively or the strand may jump out of guides, catch on projections, etc. In winding or unwinding, the strand tension should usually be substantially constant in the processing line, but it is often necessary to vary the tension at the spooler as a function of the coil diameter in order to form a good coil. Even for constant tension, torque must change with coil diameter. It is also important to be able to vary the tension to accommodate different products or for other reasons.
Another important requirement is that the drive system exhibit as smooth a transition as possible as it accelerates or decelerates between different speeds or rest. A discontinuous, jerky transition can break the strand or introduce variations in the tension which adversely affect the quality of the product. A controlled emergency stop capability is also important. These operational characteristics are particularly difficult to achieve in winding and unwinding operations for metallic strands where a full coil can weigh up to many tons, line speeds can be quite high (up to 3,000 feet per minute) and rotation of the coil at even a moderate speed produces a high degree of inertia.
In the past, a wide variety of drives and controls have been used for winders, unwinders, and other line drive elements such as pinch rolls and bridles. Known systems have used AC motors, DC motors, and hydraulic motors as the final drive element. Drive control mechanisms have included adjustable brakes, variable clutches, variable displacement hydraulic motors, as well as mechanical and hydraulic transmissions, and variable voltage, current, and/or frequency to electric motors.
U.S. Pat. No. 3,053,468 to Zernov et al, for example, describes a hydraulic drive system where a mechanical cam system senses the diameter of the roll being wound to control the rate of rotation of the drive. U.S. Pat. No. 2,677,080 describes the control of a hydraulic motor or pump through a balancing of the hydraulic fluid pressure against a set pressure. U.S. Pat. Nos. 2,960,277 and 2,573,938 disclose a solenoid operated directional valves connected in a hydraulic system for control of the system in response to an electrical signal. U.S. Pat. No. 2,988,297 describes a pneumatic system for controlling a slip clutch in the drive train of a spooler. U.S. Pat. No. 3,784,123 describes a hydraulic system where a mechanical system converts a web tension into a corresponding hydraulic pressure. A hydraulic circuit compares this pressure to a reference value. The output of this circuit controls the displacement of a hydraulic motor operating at a constant pressure to vary the output torque. This patent also discusses many of the deficiencies of other prior art tension control systems, whether mechanical, hydraulic or electrical.
Often known drive systems for winders and other process line equipment in the manufacture of metallic strand and sheet products use a regenerative, four quadrant DC motor and control ("drive"). However, this drive is large, complex, and comparatively costly. In operation, it cannot maintain a large stall tension indefinitely (even with an expensive cooling system), it cannot make a smooth, stepless transition from motoring to braking, and it does not possess extra braking torque for controlled rapid stops from high speeds.
In general, known hydraulic drive systems suffer from limited operating ranges with respect to both speed and tension, a stepped, jolting transition between motoring and braking and between different speed and tension settings on the fly, an inability to brake suddenly without jolts, and a limitation as to the controls that can interface with the system. Also, known hydraulic systems do not provide a stepless transition between speed control and tension control modes. Also, most hydraulic systems are comparatively costly and complex.
It is therefore the principal object of this invention to provide a drive and control system for winders, unwinders and other process line equipment that operates over a wide range of speeds and tensions and in a variety of modes while at the same time providing a smooth acceleration, deceleration and transition between motoring and braking, and between speed and tension control.
Another object of the invention is to provide a system with the foregoing advantages that also brakes smoothly and rapidly under emergency conditions from a high line speed to a stop even when the system is driving a high inertia load.
Another object of the invention is to provide a drive and control system that operates well in winding or unwinding coils of material having a large mass and a high rotational inertia.
Another object of the invention is to provide a drive and control system that interfaces with a variety of manual and automatic controls including computer controls, switches, relays and a variety of transducers.
Another object of the invention is to provide a drive system which can maintain a moderate to large stall tension for an indefinite period of time.
And still another object of the invention is to provide a drive system and control that automatically tapers the tension during winding and accommodates for the system inertia on acceleration or deceleration to maintain a desired tension level in the material.
Yet another object of the invention is to provide a drive and control system that is formed through a comparatively small number of components, has a relatively uncomplicated design, and has a comparatively moderate cost as compared to known drive and control systems.
A still further object of the invention is to provide an electrohydraulic drive and control system for traversing a spooler that maintains the strand being wound or payed out in a precisely predetermined lateral position.